1. Field of the Description
The present description relates, in general, to robots (e.g., humanoid or anthropomorphic robots) and design of arms for such robots, and, more particularly, to a robot arm that is designed to be passively safe through the use of counterweights to provide gravity-based counterbalancing.
2. Relevant Background
Robots are increasingly used outside the factory setting such as in surgery, patient therapy, home service, entertainment, and many other settings and applications. Often, it is desirable for these robots to be humanoid or anthropomorphic, and this may involve including two arms, with each being attached to a body via a shoulder and having human-like movements or a similar range-of-motion as a human arm.
A robot that has direct contact with humans must be designed to meet high safety standards. However, to be useful as a humanoid robot, the robot also should be designed, physically and functionally, to be compatible with the speed, dexterity, and range-of-motion of its human counterpart. For example, the robot's arms should be designed to move in a manner that matches the human arm. This need may be self-evident for rehabilitation, exoskeleton, and entertainment character-simulating robots (or animatronics), but anthropomorphic configurations of robots are also being used more and more frequently in factory or industrial settings. Often, to meet safety needs, it is desirable to design and use robots that are “passively safe.” This means that the robot's actuators are physically incapable of moving the arms in any way that can cause injury to a human. In this regard, maximum limb speed can be set by limb inertia, surface compliance, geometry, and the pressure and impulse limits specified by one or more relevant safety standards.
Excepting high-speed robot arms, gravity loads frequently dominate actuator torque budgets. For a passively safe design, overcoming gravity may consume all available torque, which limits the arm to low-speed operations. Gravity counterbalancing using either counterweights or springs can be used to allow the motors to be sized to the dynamic loads, allowing for smaller motors and reduced resting power consumption. This can be a desirable benefit for mobile applications.
Unfortunately, existing counterbalancing designs for robots including the robot arms has not proven wholly useful for anthropomorphic robots with desired arm movement. Counterweights often add significantly to arm mass when the counterweights are provided on or in the arm, and, hence, designers have generally moved away from the use of counterweights. In other cases, the counterweight is positioned on an extension element extending from the upper arm, but this arrangement is not useful for humanoid robots as the extension extends well beyond a typical human form factor and generally limits the range-of-motion. Other designs have utilized spring-based counterbalance systems, with the spring often also positioned within or on the arm. However, spring counterbalance systems are mechanically complex to implement, and the added components can actually result in no or only limited reductions in mass compared with existing counterweight designs.